1. Field of Invention
This invention is in the field of radar antenna boresight orientation.
2. Description of the Related Art
An important function of a radar system, whether a Real Beam type, Synthetic Aperture (SAR) or Interferometric SAR is to detect a target as well as identify it. Radar target detection and identification have been proven necessary in military surveillance, reconnaissance, and combat missions. The detection and identification of targets provide real-time assessment of the number and the locations of targets of interest.
One method of target detection and identification is to process the image acquired by the radar using, for example, Synthetic Aperture Radar (SAR) technology. By processing a SAR generated image, the features of a target can be extracted and matched to a database for identification.
The general principle behind SAR is to obtain high resolution images by coherently combining the amplitude and phase information of separate radar returns from a plurality of sequentially transmitted pulses from a relatively small antenna on a moving platform. The returns from the plurality of pulses transmitted during a SAR image, when coherently combined and processed, result in image quality comparable to a longer antenna, corresponding approximately to the synthetic “length” traveled by the antenna during the acquisition of the image.
High resolution SAR maps are obtained by coherently combining return signals reflected from transmitted pulses in the cross range direction from radar platform movement. However, formation of focused SAR images or maps requires accurate information on platform position and velocity to shift and focus the received radar returns over the duration of the image acquisition time, the array length, so as to have a useful, phase adjusted combination of pulse returns from multiple pulses transmitted at different times from different radar positions. The process of aligning pulses in time and space for coherent integration is referred to as motion compensation, and is usually performed with the raw radar data, at the early stage of the image formation process.
The SAR process becomes more intricate for moving targets. In order to locate moving targets precisely in range and azimuth with a single radar, accurate angle measurements using the monopulse capability of the SAR antenna need to be performed. That is, the target location needs to be determined with respect to the actual boresight of the radar antenna performing the angle measurement from boresight to the target. The accuracy of these angle measurements is often limited by bias errors internal to the radar which are not related to the signal to noise ratio of the received radar return. In effect, the bias errors preclude accurate monopulse angle measurement to a target, even if the target is clearly visible by the radar.
One cause of these bias errors is the presence of the radome in the path of both the transmitted and the received radar signal. The radome acts as a lens at the radar frequencies, thus bending the signals that pass through it. This lens is typically not constant over the extent of the radome, and the area swept by the radar transmit/receive function.
Another cause of bias errors comes from imbalances in the antenna/receiver system. A monopulse is formed from the Sum and Difference channel. However, because of channel imbalance, the gain of these channels changes as a function of elevation/azimuth. Thus, an angle error arises during the actual angle measurement, restricting the accuracy of the monopulse.
One approach of the prior art to remove these bias errors corrupting monopulse angle measurements related to radome uncertainty and channel imbalances is to conduct exhaustive calibration procedures to map these bias errors. The errors and associated corrections are accounted for separately, typically in a table. For example, the precise, a prior known, calibrated angle corrections are stored in an elevation/azimuth table, and actual measurements are corrected using entries in the elevation/azimuth table. The measurements needed to compile the entries in the monopulse error correcting table are both expensive and of limited accuracy. The error table can become inaccurate because channel imbalance can change as a normal part of radar operation, receiver/transmitter element aging and the like. This inaccuracy limits the angle measuring capabilities of the monopulse.